The physicist Richard Feynman, in a lecture to undergraduates at the California Institute of Technology in 1961, posed a question and then answered it:
If, in some cataclysm, all of scientific knowledge were to be destroyed, and only one sentence passed on to the next generations of creatures, what statement would contain the most information in the fewest words? I believe it is the atomic hypothesis (or the atomic fact, or whatever you wish to call it) that all things are made of atoms.1
The profound insight that the entire material world can be described succinctly as composed of fundamental building blocks is at the foundation of all theories about the nature of matter, from ancient inquiries into its properties, to medieval and early modern attempts to transmute base metals into precious gold, to modern efforts to understand atomic structure, harness the power of the atom with fission and fusion, and create artificial materials in laboratories.
Three new books examine our current understanding of matter’s origin and qualities, and chronicle our continuing quest to probe beyond atoms. Neutron Stars: The Quest to Understand the Zombies of the Cosmos by Katia Moskvitch, a science writer, explores recent research into the super-dense remains of stars ten times more massive than our Sun, whose precise material composition has eluded us. The astrophysicist Katie Mack’s The End of Everything (Astrophysically Speaking) shows how the contents of our universe—matter and energy—determine its destiny and, ultimately, its demise. In Fundamentals: Ten Keys to Reality, the physicist Frank Wilczek, who was awarded the Nobel Prize in Physics in 2004, addresses new discoveries that are leading to a reassessment of the atomic hypothesis. He explains how notions of matter have changed over the past decades from “all things are made of atoms” to “all things are made of elementary particles”—the expanding list of which includes quarks, gluons, muons, and the recently discovered Higgs boson.
Atomism in the West is first attributed to the fifth-century-BCE Greek philosophers Leucippus and his pupil Democritus. (The word “atom” derives from the Greek atomos, meaning “without parts.”) While none of their original writings have survived in complete form, fragments and quotations attributed to Democritus give us an idea of his atomic theories. The most famous text influenced by Democritus is De rerum natura (On the Nature of Things), the didactic poem in six books by Titus Lucretius Carus (99 BCE–55 BCE), a poet of the late Roman Republic and a follower of the Greek philosopher Epicurus, who had taken up Democritus’s atomic theory. Lucretius—who did not use the term “atom,” opting instead for “first things” and “the seeds of things”—declared that the fundamental units of matter were infinite, immutable, and invisible; constantly in motion; and subject to an occasional and unpredictable small “swerve,”2 endlessly combining or splitting apart and reconstituting themselves to take on new forms. According to Lucretius, while objects in the universe, ranging from the distant stars to life forms crawling on the Earth, are transient, their constituents—the fundamental seeds that make them up—are eternal.
In the first two books of De rerum natura, which focus on the material world, Lucretius compares these fundamental particles to letters in an alphabet, a finite set that can form an infinite variety of sentences. Just as there are letter combinations that are not permitted in a language, not all seeds can combine with all others; moreover, Lucretius asserted, there is a code that governs the permitted combinations. His account of matter, which sounds strikingly like a description from a modern-day physics or chemistry textbook, was utterly radical. Though Lucretius didn’t claim that he knew the code dictating how these seeds could arrange and rearrange themselves, he did suggest that it was knowable through investigation. This is, of course, the goal of modern science—to uncover, via observation and investigation, the laws of nature that govern the regularities and patterns in phenomena.
The concept of atoms was not confined to esoteric discussions in natural philosophy but garnered broad acceptance and percolated into the larger culture, cropping up in unlikely places, for instance in Shakespeare’s description of the fairy prankster Queen Mab in Romeo and Juliet:
She is the fairies’ midwife, and she comes
In shape no bigger than an agatestone
On the forefinger of an alderman,
Drawn with a team of little atomies
Athwart men’s noses as they lie asleep.
With his use of the phrase “little atomies,” Shakespeare reveals that by the 1590s, his world was well acquainted with the atomistic view of matter.
Atomic theories appear in other philosophical traditions too. The notion that all matter is composed of small, indivisible particles is found in the teachings of the ancient Indian analytic Vaisesika philosophical school, which dates back to the third century BCE. According to Vaisesika atomism, the four elements (earth, water, air, and fire) each come in two variations: atomic—that is, invisible, indivisible, and indestructible; and composite—that is, visible and perceptible. A Vaisesika text called The Manual of Reason argued that if there were no fundamental unit of matter, a mountain and a mustard seed would be the same size, since they would each contain an infinite number of parts.
With the development of quantum physics in the early twentieth century, scientists established that the atoms that make up all visible matter—which physicists call baryons—are composed of subatomic units: protons and neutrons, which form the atom’s nucleus, and electrons. It was soon discovered that the properties of atoms determined which elements could bond together to make new molecules, and that in certain conditions atoms could be transformed into others through fusion and fission, nuclear processes that release enormous amounts of energy.
More recently, cosmology has provided us with an inventory of the material content of the universe, and what a bizarre one it is. Every one of the ninety-four elements found naturally on Earth was created elsewhere in space, and all but three—hydrogen, helium, and lithium, which were synthesized within the first three minutes of the birth of the universe at the Big Bang—were formed in the cores of distant stars, where extreme conditions cause nuclei to collide and fuse, creating elements of greater atomic mass.
At the end of their life cycles, stars explode, dispersing carbon, silicon, sulfur, magnesium, calcium, and iron into space, enriching the hydrogen clouds from which the next generation of stars eventually forms. These elements may have arrived on Earth via meteors, or possibly were present in the matter that coagulated into our solar system 4.5 billion years ago. The calcium that our bones are made of and the iron that permeates every red blood cell in our bodies come from stars. Clichés aside, we are literally made of stardust.
In 1930 the Indian astrophysicist Subrahmanyan Chandrasekhar calculated that the birth mass of a star determines its ultimate fate: depending on its initial size, it will become either a white dwarf, a neutron star, or a black hole. A star the size of our Sun has an interior with a temperature of about 15 million degrees centigrade—so hot that atomic nuclei are stripped of their encircling electrons. These subatomic particles—electrons and nuclei—constantly collide with one another, generating pressure inside the core of the star. This internal pressure prevents gravity from causing the star’s collapse.
Pressure is also generated when hydrogen nuclei, which each have one proton, fuse with other hydrogen nuclei to form helium, which has two protons. Once all the available hydrogen in the core is exhausted, gravity starts to prevail. This causes the core of the star to contract and heat up, leading to the formation by fusion of heavier elements, such as carbon, oxygen, and silicon. Our Sun will run out of hydrogen in another five billion years or so. As its core contracts, its outer layers will expand, passing into what is referred to as the red giant phase. At this point, its radius will have grown so large that it engulfs the orbit of Mercury. It will continue to collapse, ultimately leading to the formation of a dim remnant, a white dwarf, with fusion no longer supplying energy in the core.
In 1933 the Swiss astronomer Fritz Zwicky proposed that stars more massive than our Sun would die by imploding—that is, by collapsing in on themselves due to gravity—and that this process would cause protons, which carry a positive electric charge, to capture negatively charged electrons, leading to the production of neutrons. The energy released in this process would power dramatic supernova explosions, leaving behind the neutron-rich, super-dense core—a neutron star.3 Moskvitch dedicates her book to Zwicky, does justice to his ideas, and gives him credit for predicting and detecting supernovae—cosmic beacons that have been important in shaping our view of the cosmos. Supernovae serve as standard rulers for measuring distances and were instrumental in the 1998 discovery of dark energy, which is believed to power the accelerating expansion of the universe.
Zwicky was also the first to propose the existence of dark matter, in order to explain why some galaxies appear to move faster than expected. Dark matter, believed to be crucial for the formation of galaxies, is composed of an as yet undetected particle that likely formed in the infant universe. Like every other kind of matter, it exerts and responds to gravity, but it does not interact with light, rendering it invisible and therefore extremely challenging to detect. Cosmologists estimate that dark matter makes up about 24 percent of all the stuff in the universe. By comparison, the ordinary atoms that we are made of account for a mere 4 percent. To add to the mystery, the dominant constituent of our universe is yet another invisible and immaterial entity, dark energy, comprising about 70 percent of the overall cosmic inventory. We know how both dark energy and dark matter manifest in our universe—but not quite what they are.
Neutron stars are the densest form of matter currently known, with about 1.4 times the mass of the Sun packed into a radius of just six miles. A teaspoon of neutron star material would weigh 10 million tons. Like their more enigmatic cousins, black holes, neutron stars are stellar corpses, but they come in many types: there are pulsars, which spin at extremely rapid rates, close to a thousand times per second; and magnetars, which are the strongest magnets known in nature. Unlike black holes, neutron stars possess surfaces, suffer starquakes that we can detect, and are also thought to be the source of gamma-ray bursts, the most energetic explosions in the universe.
Moskvitch offers riveting explanations of what astronomers have learned so far using radio telescopes, starting with Jocelyn Bell’s discovery in 1967 of the first pulsar, and what puzzles remain in the tantrums as well as quiet murmur of neutron stars. She opens with a wonderful account of the first-ever observation, in August 2017, of the collision of two neutron stars, by researchers working with the gravitational wave detectors LIGO in the US and Virgo in Italy. Unlike the dark collisions of two black holes and the resulting tremors in space-time—gravitational waves—that LIGO first recorded in 2016, the neutron-star collision was accompanied by visible fireworks: a bright flash of gamma radiation arrived seconds after the gravitational waves, reaching Earth after a 130-million-year journey. Researchers used this alert signal to observe the crash in other ways, using radio, optical, near infrared, X-ray, and gamma ray telescopes—opening up the new field of multi-messenger astronomy.
These multiple sets of eyes, spanning different wavelengths of light, captured the collision’s debris cloud in vivid, unprecedented detail and recorded the production of heavy elements—including an estimated 236 sextillion tons, or forty times the Earth’s mass, of pure gold. This was the first time we witnessed this process unfold. The conditions inside a neutron star are not powerful enough to create elements heavier than iron; only the collision of two neutron stars can do so. All the gold we know of, including the gold wedding ring on your finger or the chain adorning your neck, was created by collisions of two neutron stars in the distant universe.
Moskvitch closes with a description of the current frontier—recently detected fast radio bursts (FRBs) that most believe to be emitted by neutron stars, though we don’t yet have a conclusive theory to explain them. The ultimate mystery, though, pertains to the equation that describes the state of matter packed into neutron stars. As Moskvitch observes, this fundamental question is one that brings together fields in physics that developed largely in parallel—nuclear physics, condensed-matter physics, and astrophysics. In this incredibly compact state, matter and its behavior hold many more secrets. For example, the density at the center of a neutron star is thought to be so high that neutrons themselves are crushed out of existence, freeing the three quarks inside each of them. The core would then consist of a liquid of quarks, called quark matter. Quark matter might have even more peculiar properties: it is expected to be similar to the state of electrons in a metal, and perhaps even exhibit a type of superconductivity.
Contrary to Lucretius, our material world is not really indestructible or eternal. So how will it all end? In The End of Everything (Astrophysically Speaking), Katie Mack explains the possible fates, each terrible in its own way, that await our universe. The equations derived from Einstein’s theory of general relativity connect the contents, shape, and destiny of the universe. The past, present, and future are therefore determined by its evolving material and immaterial constituents. What may seem surprising, she writes, is that “much as modern cosmology informs our understanding of the very, very small, particle theories and experiments can give us insight into the workings of the universe on the largest scales.” The complex interplay of the microscopic and macroscopic determines cosmic eschatology.
As Mack points out, only one thing is certain: the universe will end. It simply cannot persist unchanged forever. The universe has been expanding since its birth about 13.8 billion years ago. As its composition has changed from being dominated by radiation for the first 30,000 years of its existence to being dominated by matter and then by dark energy (for the past 4 billion years), the expansion rate has also changed. Further transitions will determine the universe’s ultimate fate. This is a challenging question that several large teams of cosmologists are probing with observational surveys and experiments.
The five possible cosmic catastrophes that Mack discusses are the Big Crunch, in which our current cosmic expansion reverses and the universe condenses into a black hole—a singularity; the Heat Death, in which the universe expands forever, getting darker and more desolate; the Big Rip, a dark-energy driven, violent fate in which gravity is overpowered and eventually everything, including atoms, are ripped apart; Vacuum Decay, the least likely scenario, in which a rogue bubble of “true vacuum” would run amok and essentially cancel the universe; and the bounce—the most speculative of these possibilities—a cyclic cosmology where birth and death alternate repeatedly.
Though I am drawn by temperament to a cyclic universe that has no beginning and no end, the possibility that fires my imagination is the Big Crunch. The sequence would start with a slowdown of the current accelerating rate of expansion before reversal. Having flipped course, a contracting universe would become an extreme place—heating up to incredibly high temperatures and densities, beyond anything we can produce in the laboratory. None of our current theories, quantum mechanics and general relativity included, offer any guidance to the behavior of matter at such high densities. Mack writes that what “you’d encounter when the entire observable universe is collapsing into a subatomic dot are all kinds of incalculable.” Nothing material that we know of would survive; eventually it all would hurtle rapidly into a singularity. There is a strange symmetry to this fate, in which everything may end up as it was before the Big Bang—in ashes, as it were.
A book that outlines the grim fates that await our universe might seem pessimistic, but we humans are unlikely to bear witness to any of these catastrophic outcomes, as they will not manifest for billions of years. Even as Mack manages to simplify, with a disarmingly colloquial style, many complex and abstract physical concepts while explicating cosmic doom, she leads us to dream of the end without agonizing about it.
By classifying matter into “particles of construction, particles of change, and bonus particles” in Fundamentals, Frank Wilczek offers an authoritative update to Democritus’s atomic hypothesis. He shares ten profound ideas that he believes describe all of physical reality and our experience of it. As with his previous books, Wilczek deftly blends contemplative elements with a clear exposition of basic physics, adding cautious speculation about future experiments likely to reveal deeper facets of reality. Fundamentals is divided into two main sections, titled “What There Is” and “Beginnings and Ends,” with alluring chapter titles in the former section that include “There’s Plenty of Space,” “There’s Plenty of Time,” “There Are Very Few Ingredients,” “There Are Very Few Laws,” and “There’s Plenty of Matter and Energy.”
Wilczek explains the three primary properties of elementary particles from which all others can be derived—mass, charge, and spin—noting “that they are things you can define and measure precisely.” Yet although they can be measured, “the connection of the primary properties—the deep structure of reality—to the everyday appearance of things is quite remote.” While the mass and charge of the building blocks of matter are easy to understand, spin is a challenging concept for the nonphysicist. Wilczek elegantly explains how elementary particles are essentially like tiny spinning tops or gyroscopes. He summarizes “four (deceptively) easy principles” that govern how the world works, as far as we know—the basic laws of physics describe change, are universal, local, and precise.
Wilczek then outlines three big questions pertinent to our understanding of the physical world and beyond. What caused the Big Bang? Are there even more meaningful patterns hidden in the sprawling landscape of fundamental particles and forces that we have not uncovered so far? And what is the nature of consciousness—did mind emerge from matter, and if so, how? While his philosophical speculations on the nature of consciousness and the emergence of complexity meander, he is especially crisp about the mysteries that remain to be solved in physics.
In particular, he introduces axions—hypothetical subatomic particles whose name he coined—as the link that may help unlock two mysteries that appear unrelated: the unknown nature of dark matter and the near-exact temporal symmetry of known physical laws. Physical laws appear to almost retain their form independent of the direction of the flow of time—this is a real puzzle as we live in a universe in which time moves in only one direction. Physicists suspect that this signals the existence of a new kind of particle—the axion—that formed in the early universe and has the right properties to be dark matter. Wilczek is enthusiastic about the many experimental efforts currently underway to detect the axion. With so much known and understood about the nature of ordinary atoms, we still eagerly await the discovery of new classes of subatomic particles that might hold the key to many vexing theoretical problems.
I write this soon after the announcement from the Fermi National Accelerator Laboratory in mid-April of a tantalizing development in the subatomic realm: a mismatch between theoretical computations and experimental measurement of the wobble of a subatomic particle—the muon, a heavier cousin of the electron—in a magnetic field.4 If this holds up, it portends the existence of a fifth fundamental force in nature—along with gravity, electromagnetism, and the strong and weak forces—as well as the existence of new subatomic particles. This would completely shake up our understanding of matter. The remarkable thing about the era we live in, which Wilczek aptly characterizes as a time when “technology has already given us superpowers, and there is no end in sight,” is that radical, transformative discoveries like this one, which could dramatically alter our concept of the universe, might be just around the corner.
July 1, 2021
As American as Family Separation
The text and original audio recordings of the Feynman Lectures on Physics are available at feynmanlectures.caltech.edu. ↩
For a gripping account of the discovery of a copy of De rerum natura in a Benedictine monastery in Fulda by the Italian scholar Poggio Bracciolini in 1417, see Stephen Greenblatt, The Swerve: How the World Became Modern (Norton, 2011). ↩
Using Einstein’s famous formula E=mc2, Zwicky calculated that the mass lost during the star’s collapse would be converted into energy, which powers the supernova explosion. ↩
The mismatch was not deemed significant enough to be reported as a “discovery,” but further experiments may yield greater discrepancies. See “First Results from Fermilab’s Muon g-2 Experiment Strengthen Evidence of New Physics,” fnal.gov, April 7, 2021. ↩